Embodiments of the present invention are concerned with an apparatus and/or a method for creating a multi-dimensional representation of an object, and, for example, they deal with how computer tomography image reconstruction methods may be employed without performing any rotational motions of the apparatus or of the object to be examined in small angular increments.
In computer tomography methods that are employed, for example, for diagnoses on human bodies or for testing materials in the industrial sector, an object to be examined is typically X-rayed by an X-ray source, wherein a sensor arranged opposite the X-ray source in relation to the object to be examined creates X-ray photographs of the object X-rayed. When employing an area sensor, the pictures thus created are two-dimensional, when employing a line sensor, they are one-dimensional. A multi-dimensional representation, i.e. for example a three-dimensional or a two-dimensional representation, is enabled in that the perspective from which the object is illuminated, or from which pictures of the object are taken, is changed. This involves taking pictures of the object from several perspectives. By changing the perspective, i.e. for example the angle from which the object to be examined is illuminated, information about an additional dimension, specifically depth information, i.e. positional information in a direction to the sensor surface, may be obtained from the projection pictures.
In diagnostic methods, it is customary to rotate the arrangement consisting of the X-ray source and the detector about a patient located at the center of the arrangement. In material testing methods, the object itself is often rotated, while the arrangement consisting of the sensor and the X-ray source remains stationary. To enable complete three-dimensional or multi-dimensional reconstruction of the object with the maximum spatial resolution available, tone may take pictures of projections of the object from a solid-angle range or from a plane-angle range of 180° in parallel-beam geometry. The angular increments of adjacent pictures also depend on the desired achievable spatial resolution, which is typically limited, in terms of hardware, by the resolution of the sensor used and by the precision of the mechanics.
In industrial material testing tasks, the objects to be examined are often so large that rotation of the detector arrangement about same or rotation of the object itself are out of the question for mechanical or cost reasons.
Even if rotation is possible, non-destructive testing of objects by means of the rotation-dependent computer tomography methods of 3D X-ray computer tomography, the achievable spatial resolution is frequently limited by mechanics. This results in a degradation of the spatial resolution, which may be several orders of magnitude higher than the actual achievable maximum spatial resolution, which is limited by the intrinsic sensor resolution of the sensor used. This results from the fact that for a full three-dimensional representation, tone may rotate the object or the apparatus relative to said object so as to measure projections from directions of at least 180°+plane angle. Particularly for large objects (e.g. freight containers or freight vehicles comprising containers), the mechanical rotational motion—even if it is possible, in principle—may be difficult, since large weights and/or large spatial dimensions may be moved with high precision and high repeating accuracy. This places extremely high demands on the adjustment and the precision of the axes used for mechanical movement.
Similar considerations apply to objects having unfavorable contours or envelopes that strongly deviate from the rotational symmetry (such as wings of airplanes or printed circuit boards, for example). With such flat, extensive objects, the distance between the focal spot of the X-ray source and the rotational axis cannot be selected to be arbitrarily small because said distance may be at least large enough so that the object, in its most unfavorable geometric extension, can be rotated or moved past the X-ray source. This results in a geometry-related limitation of the achievable resolution, it being possible for said resolution to be improved by optically magnifying the imaged elements of the object in that the source is brought close to the object to be examined, so that in accordance with the laws of ray optics, a magnified representation of the object is imaged on the sensor.
U.S. Pat. No. 6,814,489 B2 shows a rotational CT wherein the X-ray source and the sensor may be radially adjusted in relation to the object to be examined so as to set a magnification in the projection pictures produced by the apparatus.
The Japanese Publication “Study of New Linear Movement Tomography Using Magnification Ratio” describes a method wherein several X-ray photographs are taken of an object to be examined, while both the X-ray source and the X-ray sensitive sensor are moved relative to the object to be examined. Once the images have been corrected with regard to the different magnification ratios, the images thus obtained are added, said addition causing a blurring effect with regard to those picture elements that are not located within a plane in the object that is dependent on a displacement of the source and of the sensor. Thus, a two-dimensional picture is created by means of simple addition, wherein the contours of the objects in an advantageous plane are sharper than the contours of the objects in other image planes.
U.S. Pat. No. 5,095,501 proposes a CT method wherein use is made of two pairs of X-ray source and X-ray detector which are arranged at right angles to each other, so that the examination time for the patient may be halved.
European patent Application 1 627 601 A1 deals with a method of how stray artefacts caused by metallic implants within a patient's body may be suppressed by suitable image processing.